1. Field of the Invention
The present invention relates to an output voltage adjusting method for laser power monitoring whereby adjustment is performed in an optical pickup provided in an optical disk recording and reproducing apparatus by changing an external variable resistance for output voltage adjustment provided in a laser power monitor circuit so that the output voltage of the laser power monitor circuit is made equal to a predetermined value with a laser diode operating at a predetermined laser power.
2. Description of Related Art
Information is recorded on writable optical disks such as CD-Rs (Compact Disk-Recordable) or DVD-Rs (Digital Versatile Disk-Recordable), or rewritable optical disks such as CD-RWs (Compact Disk-Rewritable) or DVD-RWs (Digital Versatile Disk-Rewritable) in such a way that, for example, recorded pits are formed by irradiating an optical disk coated with an organic dye base recording material applied thereto with laser light. For the stable formation of recorded pits in such an optical disk under given conditions, it is necessary to control a driving current of a laser diode (hereinafter referred to as an LD) so as to obtain a constant laser power.
In optical disk recording and reproducing apparatuses, data is recorded as follows. For example, for CD-Rs and DVD-Rs, data is recorded thereon by irradiating a recording film formed on a disk with laser light of high intensity emitted from an LD and thus forming pits on the recording film by thermal reaction. For CD-RWs and DVD-RWs, data is recorded thereon by changing the crystalline state of the recording film. On the other hand, data recorded on the optical disk is reproduced by reading reflected light resulting from laser light of low intensity emitted from the LD and shone onto the recording film.
Incidentally, in an optical pickup provided in the optical disk recording and reproducing apparatus, adjustment is performed by monitoring the laser power of laser light emitted from the LD with a laser power monitor circuit and then feeding back the result thus obtained to a laser drive control circuit so that the laser power of the LD remains constant.
FIG. 5 is a block diagram showing the circuit configuration of a typical optical pickup. In FIG. 5, an optical pickup circuit 9 has an LD 1, an optical system 2, a laser power monitor circuit (which is an integrated circuit also called a front monitor PD circuit) 3 including a PD (that is, a photodiode) 4, a I/V (current/voltage) conversion circuit 5, and an amplification circuit 6, and a laser drive control circuit 7.
In order to perform an operation such as recording on or reproducing from an optical disk 8, the optical pickup circuit 9 first feeds, to the LD 1, a laser driving signal needed for such an operation from the laser drive control circuit 7. The LD 1 emits a laser optical signal having a laser power corresponding to the laser driving signal received from the laser drive control circuit 7. Much of the optical signal thus emitted is shone onto the optical disk 8 via the optical system 2; part thereof is incident on the PD 4 of the laser power monitor circuit 3. Then, a current flowing through the PD 4 is converted into a voltage by the I/V conversion circuit 5, and then is amplified to a voltage having a predetermined level by the amplification circuit 6. The resultant voltage is then fed back to the laser drive control circuit 7. This allows the laser drive control circuit 7 to adjust the voltage of a laser driving signal to be fed to the LD 1 based on the voltage fed back from the laser power monitor circuit 3.
The problem here is that the output voltage of the laser power monitor circuit 3 differs from one apparatus to another, and the offset present in the circuit such as the offset of the PD 4, if large enough, affects the laser power of the LD 1.
To address the problem described above, with a conventional output voltage adjusting method for laser power monitoring, adjustment is performed by measuring the laser power of laser light emitted from the LD 1 with an optical power meter and changing an external variable resistance for output voltage adjustment (for example, VR1 and VR2 shown in FIG. 3) while observing the output voltage of the laser power monitor circuit 3 with an oscilloscope so that the output voltage of the laser power monitor circuit 3 is made equal to a predetermined value with the LD 1 operating at a predetermined laser power.
For example, assume that adjustment is performed so that the output voltage of the laser power monitor circuit 3 is made equal to 15 mV with the LD 1 operating at a laser power (i.e., a light-emitting power) of 1 mW. Then, before adjustment, when the laser power is 0 mW (that is, the LD 1 is turned off), the output voltage (unadjusted FSPD output) of the laser power monitor circuit 3 is, for example, at a voltage level indicated by a line 61 shown in FIG. 6; after adjustment, when the laser power is 1 mW (that is, the LD 1 is turned on), the output voltage (adjusted FSPD output) of the laser power monitor circuit 3 is, for example, at a voltage level indicated by a line 62 shown in FIG. 6.
However, in actuality, since the laser power monitor circuit 3 has an offset voltage of ±15 mV as required according to the specifications of the laser power monitor circuit 3 (for example, the specifications of an integrated laser power monitor circuit 3), the adjusted output voltage of the laser power monitor circuit 3 is as follows. When the offset voltage is −15 mV, the adjusted output voltage is 0 mV; when the offset voltage is +15 mV, the adjusted output voltage is 30 mV. Additionally, the offset voltage varies as a result of adjustment within a range according to the specifications of the laser power monitor circuit 3. As a result, when the laser power is 1 mW, the adjusted output voltage of the laser power monitor circuit 3 will vary up to a maximum of 45 mV. The problem here is that an optical pickup built with the laser power monitor circuit 3 adjusted in a manner as described above suffers from poor accuracy, for example, in recording and reproducing, causing inconvenience in practical use.
With a conventional technique disclosed in JP-A-2005-56965, the magnitude of the offset to be produced in response to a given noise amplitude is determined previously so that the offset voltage value is set according to an actual noise amplitude. This technique, however, dose not suggest any linkage to an output voltage adjusting method for laser power monitoring whereby adjustment is performed by changing an external variable resistance for output voltage adjustment provided in a laser power monitor circuit so that the output voltage of the laser power monitor circuit is made equal to a predetermined value with an LD operating at a predetermined laser power.
With another conventional technique disclosed in JP-A-2004-146050, the gain of a monitor PD that monitors an optical power (that is, a laser power) of an LD is controlled. This technique too, however, does not suggest any linkage to an output voltage adjusting method for laser power monitoring whereby adjustment is performed by changing an external variable resistance for output voltage adjustment provided in a laser power monitor circuit so that the output voltage of the laser power monitor circuit is made equal to a predetermined value with an LD operating at a predetermined laser power.